The production and metabolism of steroid hormones by the mammalian preimplantation embryo and their physiological significance in implantation are presently controversial. The objectives of the proposed research are aimed at these two fundamental concepts. We have recently shown that rabbit blastocysts have considerable aromatase activity (estrogen forming capacity). However, only 28% of this activity is expressed as estradiol-17Beta (E2), suggesting formation of other estrogens. One such candidate could be catechol estrogen. Recent reports indicate that catechol estrogens have uterotrophic as well as positive and negative feedback effects upon the hypothalamic-hypophyseal axis and are more potent than E2 in stimulating prostaglandin (PG) synthesis in the rat uterus in vitro. PGs are considered to participate in implantation. Therefore, we propose to investigate whether primary estrogens, can be converted to catechol estrogens in the embryo and/or the endometrium and whether these catechol estrogens can stimulate PG synthesis in the embryo and/or the endometrium. Experimental methods to be employed include culture of embryos, biochemical assays of enzymes and radioimmunoassay of PGs.